1. Field of the Invention
This invention relates to a method for purifying phosphoric acid obtained with sulfuric acid acting on natural phosphates, which comprises extracting said phosphoric acid with an ether-containing solvent, thus forming an aqueous phase containing the major part of the impurities and an organic phase containing purified phosphoric acid, and separating both said phases.
The phosphoric acid prepared with sulfuric acid acting on natural phosphates is normally obtained at the outlet from that filter which separates the residual gypsum as an impure solution containing 30 to 35% P.sub.2 O.sub.5. Said acid is then concentrated by vacuum-evaporating to a P.sub.2 O.sub.5 content in the range from 52 to 55% and it is sold commercially in such a form.
A typical analysis for such an acid is, for example, the following:
P.sub.2 O.sub.5 52% F 0.6% SO.sub.3 1.5% Fe.sub.2 O.sub.3 0.5% Al.sub.2 O.sub.3 0.6% CaO 0.1%
2. Prior Art
In known methods of the kind described in U.S. Pat. No. 3,318,661 and Belgian Pat. No. 661,743, of isopropyl ether is used as a solvent for extracting P.sub.2 O.sub.5 from a concentrated phosphoric acid and to purify same.
The advantage of isopropyl ether as solvent lies in the solubilizing power thereof for the phosphoric acid which is substantially zero below a concentration level which is in the range of 45% P.sub.2 O.sub.5 and which reaches 100% for an acid with more than 60% P.sub.2 O.sub.5. Moreover, such a solvent has a negative temperature coefficient as regards the solubilizing power thereof for P.sub.2 O.sub.5. That is, at low temperatures this solvent extracts substantially more P.sub.2 O.sub.5 than at higher temperatures. The impurity-distributing ratio between the extracted organic phase and the residual acid phase in this solvent is very advantageous since very few impurities go over to the organic phase together with the P.sub.2 O.sub.5, and this is substantially less than for an alcohol-type solvent having 5 or 6 carbon atoms.
With such a solvent it is thus possible to extract high amounts of P.sub.2 O.sub.5 from an acid in the cold with a high enough concentration and to recover the P.sub.2 O.sub.5 from the complex phase which is thus formed simply by raising the temperature thereof, and this can be done in combination with a slight water addition. The phosphoric acid is thus extracted again with a high concentration, by means of a single operation comprising contacting the organic phase and the extracted phase. Moreover, the ether solubility is very low in the extracted acid and in the residual acid, and the latent vaporization heat thereof is also very low. Therefore, it is very easy and economical to recover this solvent by conventional means. These extraction methods based on the use of such a solvent are as advantageous as they are simple and economical and these methods provide well purified and concentrated phosphoric acids at low cost.
However, said known methods have important drawbacks. Thus, to obtain a large P.sub.2 O.sub.5 extraction, said methods require the use of very concentrated phosphoric acids.
For example, from an acid with 55% P.sub.2 O.sub.5, at 10.degree.C it will not be possible to extract more than 72% P.sub.2 O.sub.5. It will thus be required to obtain a larger extraction of P.sub.2 O.sub.5, to concentrate the acid above 55%. This is however a very costly requirement as generally the output capacity of concentrating installations is very severely lowered when it is desired to go above a concentration of 55% P.sub.2 O.sub.5.
It is also to be noticed that the known methods of extraction with isopropyl ether are generally very unsuitable for obtaining a high-purity acid and they are mostly used for a preliminary purifying.